What are Fizeau’s fringes?

Fizeauâ€™s fringes occur in interferometry. One of the most common methods used to test the flatness of a polished surface is by analysing the interference patterns formed when the surface is placed against another polished flat transparent surface. When two surfaces are fully in contact, a pattern of concentric dark and light circles is seen and these patterns (or fringes) are called Newton’s rings. However, when the surfaces are separated by a very thin wedgeshaped layer of air, straight, parallel, dark and light patterns are produced and these are called Fizeauâ€™s fringes. These fringes are named in honour of French physicist Armand Hippolyte Louis Fizeau (1819-1896), who used the interference of light to measure the dilation of crystals.